Imaging systems

ABSTRACT

An imaging system comprises an image-data collection station ( 1 ) having a platform ( 2 ) which is rotatable about an azimuthal axis. Several image sensors ( 3 ) are mounted on and rotate with the platform ( 2 ). A static monitoring station ( 7 ) includes a computerized sensor control system ( 10 ) for selectably and independently turning the sensors ( 3 ) ON and OFF during the rotation of the platform ( 2 ). Station ( 7 ) additionally monitors the data output from the sensors ( 3 ) and delivers a visible output at a VDU ( 8 ).

The present invention relates to imaging systems and in particular toimaging systems capable of acquiring image data over a wide-angle field.

For security and other purposes there is often a requirement for asurveillance system which can be used to examine a wide-angle field ofview, often up to a full 360° panorama. GB2157526 describes an imagingsystem having a platform which is rotatable about an azimuthal axis andwhich carries an image data collection sensor having a fixed field ofview. The platform is continuously rotated about the axis to scan thesensor field of view around a 360° panorama at a fixed elevationalangle. Normally, data is acquired between two angular positions fordisplay at a display station. There exists a trade-off between thespacing of these two angular positions and the resolution of the dataacquired and displayed.

A problem with the system of GB2157526 is the lack of flexibility andthe lack of redundancy provided.

It is an object of the present invention to provide an imaging systemwhich overcomes or at least mitigates disadvantages of the above system.

According to a first aspect of the present invention there is providedan imaging system comprising an image-data collection station which isrotatable about an azimuthal axis, the image data collection stationcomprising at least two circumferentially spaced apart image sensorsarranged for rotation with the station and sensor control means forselectably and independently turning the sensors on and off during arotation of the data collection station.

It will be appreciated that, as the first aspect of the presentinvention provides a plurality of image sensors, embodiments of thepresent invention have a degree of built-in redundancy, e.g. if onesensor fails another sensor can still be used to collect image data froman area of interest. In the event that one or a proportion of the imagesensors fail, the result will not be a complete loss of image data.Embodiments also provide for increased flexibility. For example, wherethe bandwith (or capacity) of one sensor is fully used to look at afirst selected area, a second sensor can be used to look at a secondarea or to provide a higher update rate in part or all of the firstselected area.

According to a second aspect of the present invention there is providedan imaging system comprising an image-data collection station which isrotatable about an azimuthal axis, the image data collection stationcomprising at least two circumferentially spaced image sensors arrangedfor rotation with the station, wherein at least one of the image sensorshas a variable field of view and/or comprises means for varying theangle of elevation of the sensor.

Embodiments of the second aspect of the present invention enable thesensors to follow the same or different elevational paths around theazimuthal axis. This may be achieved by providing means for varying theelevation angle of a sensor. Where the paths followed by two imagesensors are the same and the sensors have the same field of view, thedata from the two sensors can be combined to provide enhanced imageresolution over an entire captured image. Where the elevational pathsfollowed by the two sensors differ, the image data provided by thesensors may be displayed on separate displays or may be combined toprovide a wider overall field of view on a single display.

In one embodiment, at least one of the sensors comprises a zoom lenswhich can be zoomed-in on an area of particular interest. The resultingimage data can either be displayed independently of image data capturedby other sensors or can be used to selectively enhance the resolution ofa region of an image captured by another image sensor having a widerfield of view.

Preferably, the imaging system comprises an image monitoring stationhaving control means for independently controlling the magnification ofimage sensor zoom lenses and/or the elevational angle of the imagesensors. The control means comprises a tracking computer for identifyinga target within an image and for direction one or more of the imagesensors to track the identified target by dynamically controlling theelevation angle and/or zoom of the sensors. The control means isarranged to combine image data received from the or each tracking imagesensor and from a further image sensor having a wider field of viewwhich contains the target, whereby an image can be obtained in which thetarget is shown with an increased resolution relative to the remainderof a larger field of view image.

Preferably, the image sensors are spaced at regular angular intervalsaround the image data collection station.

Preferably, the image data collection station comprises at least fourimage sensors.

it will be appreciated that a particularly advantageous imaging systemis achieved by combining both the first and second aspects of thepresent invention.

For a better understanding of the present invention and in order to showhow the same may be carried into effect reference will now be made, byway of example, to the accompanying drawings, in which:

FIG. 1 shows in plan view data collection station embodying the presentinvention; and

FIG. 2 shows a side view of the data collection station of FIG. 1including an associated control system.

In the drawings, an imaging system comprises a data collection station 1having a circular platform 2 on which are mounted four individual imagesensors 3. The image sensors 3 are arranged around the periphery of theplatform 2 is 2, spaced apart by 90°. The rotating platform nominallyarranged to lie in a horizontal plane when in operation so that theplatform rotates about a vertical, or azimuthal, axis. The system may bemodified however to utilise a two-dimensional array of sensing elements,arranged along a plurality of vertical lines. Each line is scanned inturn, with the revolution rate of the platform 2 and the sensing elementscan rate being synchronised to provide for contiguity of the imagefield over 360°.

FIGS. 1 and 2 illustrate the case where the images sensors 3 each have afield of view 4 which is substantially one-dimensional, extending over amaximum angle β in a vertical plane. Each sensor 3 typically comprises asingle column of vertically aligned image sensing elements (not shown inthe Figures).

The following description of the operation of the imaging systems ofFIGS. 1 and 2 assumes, in the first instance, that it is only requiredto obtain image data from two dimensionally opposed image sensors 3 a, 3c and that both of these sensors are arranged to have the same field ofview (i.e. β is the same for both sensors) and the same elevationalangle.

The sensor carrying platform 3 is rotated at a constant known rate by adrive mechanism 5 and accordingly the field view of view 4 of eachsensor moves in a circular locus and traverses a 360° wide field. Theabsolute angular position of the platform at any instant is determinedby an angular position sensor 6. A static image monitoring station 7comprises a video display unit (VDU) a which is capable of displaying aportion of the visual field extending over an angle α₁ of azimuthalextent and an angle β₁ in elevational extent (where β₁ is less than orequal to β). A manually-operable selector or ‘pointer’ 9 coupled to asensor control computer 10 functions to designate an angle θ, relativeto a datum direction 11 (see FIG. 1), which defines the centre of thefield of extent α₁, which the operator wishes to be displayed on the VDU8. The sensor control computer 10 also combines parameters θ and α₁ todetermine the range of angular positions, namely from (θ−½α₁) to(θ+½α₁), over which image data must be collected from the selected imagesensor(s) 3. In accordance with the calculated angular positions, thecontrol computer 10 via line 11 turns the sensors 3 on and off for theappropriate periods of their rotation. It is noted that this arrangementeliminates unnecessary power consumption and electronic noise generationby turning sensors off when it is not required to obtain image data fromthose sensors.

When a sensor 3 is on, image data from that sensor 3 is passed to anassociated gate device 12 which is also controlled by the controlcomputer 10. The gate devices 12 b, 12 d associated with the twounselected sensors 3 b, 3 d are maintained permanently off whilst thosegates 12 a, 12 c associated with the two selected sensors 3 a, 3 c aretoggled on and off substantially in synchrony with the selected sensors.More particularly, gate 12 a is turned on for the period during whichsensor 3 a is within the selected azimuthal range and off for the periodwhen sensor 3 a is outside that range. Similarly, gate 12 c is on whensensor 3 c is within the selected range and off for the remainder of theplatform revolution.

Image data passed by the gates 12 a, 12 c is checked alternately into animage framestore 13 so that the framestore is refreshed twice perplatform revolution. Data is clocked out of the framestore 13 to refreshthe VDU 8 at an appropriate rate. The framestore 13 may be of the typewherein the data is clocked-out to the VDU 8 at TV raster scancompatible rate irrespective of the data clocking-in rate which isdetermined by the characteristics of the image sensors 3 and therevolution rate of the platform 2 (a process known as ‘scanconversion’).

It will be apparent that the refresh rate of the VDU 8 is double that ofthe conventional system for the same rate of rotation. It will also beapparent that the refresh rate may be increased still further by turningon a further one or two image sensors 3. In the event that one of thesensors 3 is damaged so that it is unable to supply accurate image datato the image monitoring station 6, the system remains capable ofsupplying usable data to the VDU 8 because one or more sensors stillremain operational. The system therefore provides for a considerabledegree of redundancy with a graded performance degradation.

Consider now that one of the selected image sensors 3 is provided with azoom lens which enables a narrow region of the surrounding scene to beviewed by that sensor with increased resolution. In addition, considerthat sensor 3 is provided with a tilting mechanism to enable theelevation of the sensor 3 to be varied. Using the manually operableselector 9, an operator may select a particular area of the surroundingscene for display at a higher resolution. The system may comprise asecond image monitoring station, with a second VDU, to allow the widearea field of view to be displayed at the same time as the selected viewfield of view. This arrangement would be useful for example in asituation where a target is present within the overall field of view andan operator requires to view the target in detail whilst also retaininga view of the surrounding area (e.g. to see if other targets enter thevicinity).

Typically, all four image sensors 3 are provided with independent zoomand tilt facilitate to maximise the flexibility of the system. It istherefore possible to assign for example two image sensors 3 to thetarget whilst assigning the remaining two sensors 3 to the wide area sothat both may be displayed at a relatively high resolution.

As a target moves across the scene, it may be difficult for an operatorto continue to manually track the target. The control computer 10 maytherefore be used to automatically follow an operator-designated targetacross the scene by carrying out appropriate image processingoperations. The control computer 10 can then be arranged to control theimage sensor or sensors assigned to the target, varying the zoom andelevation as appropriate, to track the target. Provision may also bemade for predicting the trajectory of the target when it leaves thefield of view of one of the tracking sensors so that the followingtracking sensor can be set to catch the target when it enters thatsensors field of view.

The control computer 10 may also be arranged to switch on and off imagesensors 3 as required in order to optimise the tracking accuracy and thebandwidth of image data. For example, in order to track a relativelyslow moving target, only a single image sensor 3 may be required whilstin order to track a fast moving target all four image sensors may haveto be used.

It will be appreciated that various modifications may be made to theabove described embodiments without departing from the scope of thepresent invention. For example, the sensors 3 may be arranged atirregular angular spacings around the periphery of the circular platform2. The number of sensors 3 may also differ, for example the datacollection station may be provided with 2, 6, 8 or more sensors 3.

What is claimed is:
 1. An imaging system comprising: an image-datacollection station (1) which is rotatable about an azimuthal axis, theimage-data collection station (1) comprising at least twocircumferentially spaced-apart image sensors (3 a, 3 b, 3 c, 3 d)arranged for rotation with the station (1) so as to include a commonfield of view (4), each sensor comprising a single column of verticallyaligned sensing elements, and a static image-monitoring station (7)comprising a video display unit (8) for displaying image data receivedfrom the collection station (1), and control means (9, 10, 11) forselectively and independently turning the sensors on and off duringrotation of the data collection station (1), wherein image data fromeach image sensor (3 a, 3 b, 3 c, 3 d) is delivered to an imageframe-store (13) via an associated gate device (12 a, 12 b, 12 c, 12 d)which is on/off controlled in synchrony with the sensors (3 a, 3 b, 3 c,3 d) by said control means (9,10,11), and data from the imageframe-store (13) is clocked out at a TV raster-scan compatible rate tothe video display unit (8).
 2. An imaging system as claimed in claim 1,wherein at least one of the image sensors (3 a, 3 b, 3 c, 3 d) has avariable field of view and/or comprises means for varying the angle ofelevation of the sensor.
 3. An imaging system as claimed in claim 2,wherein at least one of the sensors (3 a, 3 b, 3 c, 3 d ) comprises azoom lens which can be zoomed-in on an area of particular interest. 4.An imaging system as claimed in claim 2 , wherein the control means (9,10, 11) is arranged for independently controlling the magnification ofimage sensor zoom lenses and/or the elevational angle of the imagesensors.
 5. An imaging system as claimed in claim 4, wherein the controlmeans (9, 10, 11) comprises a tracking computer for directing one ormore of the image sensors (3 a, 3 b, 3 c, 3 d) to track a target withinthe image by dynamically controlling the elevation angle and/or zoom ofthe sensors.
 6. An imaging system as claimed in claim 5, wherein thecontrol means (9,10,11) is arranged to effect combination of image datareceived from the or each tracking image sensor and from a further imagesensor having a wider filed of view which contains the target, wherebyan image is obtained in which the target is shown with an increasedresolution relative to the remainder of a larger field of view image.